This invention relates in general to optical communication devices, and in particular, to a fiber optic non-reciprocal device which circulates one or more light beams from port to port.
Optical circulators have wide applications. They are used, for example, to convert an existing unidirectional fiber optic communication link to a full duplex communication link by installing an optical circulator at each end of the link. Optical circulators are also used ill fiber amplification systems, wavelength division multiplex (WDM) networks, optical time-domain reflectometers (OTDRs) and for test instruments.
The various applications of optical circulators are described by Jay S. Van Delden in xe2x80x9cOptical Circulators Improved by Directional Fiber Systems,xe2x80x9d Laser Focus World, November 1995, pp. 109-112. Taken essentially from this article by van Delden, FIGS. 1A, 1B illustrate the operation of optical circulators. As shown in FIG. 1A, an optical signal from transmit station TxA is transmitted in a forward path through a circulator at terminal A through an optical link 1 to a receive station RxB through a second circulator at terminal B without the signal being transmitted to the station TxB. An optical signal may be transmitted from station TxB through the circulator at terminal B through the same optical link 1 to a station RxA through the circulator at terminal A in a reverse path, using the same optical link that is used in the forward path, without also transmitting the signal to TxA.
As shown in FIG. 1B, an input optical signal at 1550 nm at port 1 is input to the circulator and transmitted through port 2 and an optical link to a dichroic beamsplitter having a maximum reflection at 1550 nm and maximum transmission at 980 nm. Such signal is therefore reflected, passes through the same optical link and again reaches port 2 of the circulator and is transmitted to port 4 whereupon the signal is amplified. A 980 nm optical signal is applied by a pumping diode rough the dichroic beamsplitter, is passed through the optical link and transmitted from port 2 to port 3 of the circulator to a second amplifier.
Various optical circulators have been used or proposed. U.S. Pat. No. 5,471,340 describes an optical circulator. Such circulator is sensitive to misalignment and has high loss due to the long optical path, and is, therefore, difficult to construct and use.
In xe2x80x9cHigh-Isolation Polarization-Insensitive Optical Circulator for Advanced Optical Communication Systems,xe2x80x9d Journal of Lightwave Technology, Vol. 10, No. 9, September 1992, pp. 1210-1217, Koga et al. describe an optical isolator employing three or more birefringent crystals in an optical array of elements. The one or more birefringent crystals in the center of the array of optical elements in Koga et al.""s optical circulator are used to cause light beams in the reverse direction to deviate from the forward path to direct such beams along a different path. See, for example, FIG. 5(b) on page 1212. To accomplish the function of a circulator, light beams in the reverse path of circulator must deviate adequately from the forward path. However, since the amount of deviation through the circulator depends on the one or more birefringent crystals in the center, and since light is caused to deviate gradually along optical paths through such crystals, such crystals must have adequate optical path lengths. This causes the circulator to be bulky and to have high loss because of the long optical path. In many optical applications, compact devices are called for because of the limited amount of space available.
From the above, none of the devices proposed or in use are entirely satisfactory. It is, therefore, desirable to provide optical circulators with improved characteristics.
The invention relates to an optical circulator with non parallel interfaces which directs light from port to port. The circulator of the present invention uses a non parallel interface to route light travelling in a forward direction from a first port to a second port while the same non parallel interface routes light travelling in a reverse direction from the second port to a third port. The non parallel interfaces allow for integration of the first and third port so that some of the optical components may be shared by the ports, although the invention may also use ports having completely separate components.
A first embodiment of the invention is exemplified by an optical circulator for transmitting light circularly between input/output ports, comprising a first optical path from a first optical port to a second optical port; a second optical path from the second optical port to a third optical port, wherein a first beam travels along the first optical path and a second beam travels along the second optical path such that the direction of travel of the first beam is opposite of the direction of travel of the second beam; an optical interface in the first and second optical paths that passes the first beam but deflects the second beam to a highly reflective coating, wherein the plane of the highly reflective coating is offset from the plane of the optical interface such that it is not parallel with the optical interface and such that the second beam is deflected from the highly reflective coating to the third port, and wherein the first port and the third port utilize the same GRIN lens. Additionally, the center of the optical interface ranges from about 0.5 to about 3.0 mm from the center of the highly reflective coating. The offset of the plane of the highly reflective coating from the plane of the optical interface is about 2.0 degrees to 5.0 degrees, preferably about 2.4 to 3.3 degrees.
Another embodiment is exemplified by an optical circulation method comprising dividing a light beam from a first port into two substantially orthogonally polarized beams along two different paths, the first port comprising a first lens; causing the two substantially orthogonally polarized beams from the first port to have a fit polarization state; passing the beams having the first polarization state substantially without deflection; combining the two passed light beams originally from the first port into one beam along the same path to a second port, the second port comprising a second lens; dividing a light beam from the second port into two substantially orthogonally polarized beams along two different paths; causing the two substantially orthogonally polarized beams from the second port to have a second polarization state; deflecting at an optical interface the beams having the second polarization state; combining the two deflected beams originally from the second port into one beam along the same path to a third port using the first lens of the first port.
Yet another embodiment involves an optical circulator for transmitting light circularly between input/output ports, comprising first means for receiving a first beam at a first port and transmitting a second beam at a third port; second means for receiving the second beam and transmitting the first beam at a second port; first means for dividing the first beam into two substantially orthogonally polarized beams and for combing the second beam along two different paths into one beam along the same path; second means for dividing the second beam into two substantially orthogonally polarized beams and for combining the first beam along two different paths into one beam along the same path; spatial separation means having a first optical interface in a path or paths between the first and second means for dividing, the spatial separation means causing the first beam to pass substantially without deflection and for causing the second beam to be deflected at said first optical interface to a second optical interface, wherein said second optical interface deflects the second beam to the third port of the first means such that the first means simultaneously receives the first beam at the first port and transmits the second beam at the third port; and means for causing two substantially orthogonally polarized beams from the first beam to have the first polarization state when they reach the spatial separation means and for causing two substantially orthogonally polarized beams from the second beam to have the second polarization state when they reach the spatial separation means.